1. Field of the Invention
This invention generally relates to methods and systems that generate and process electromagnetic signals in the radio frequency, microwave and optical frequency spectra. More specifically, the invention relates to such methods and systems that modulate these electromagnetic signals and prepare the modulated signals for transmission over significant distances. Even more specifically, the present invention relates to such method and systems that are particularly well suited for enabling such electromagnetic signals to carry data at a rate of forty gigabits and above for long distances.
2. Description of the Prior Art
The demand for bandwidth has driven the introduction of higher data rate protocols such as ten Gigabit Ethernet technology and Synchronous Optical Networks (SONET). Serial links at this data rate are limited by dispersion to a few km's on singlemode fiber; the next generation of forty Gbit/s and beyond data links will face even more severe distance limitations. Because this limit is caused by dispersion, not link budget, it cannot be overcome by a brute force approach of increasing the laser power. In any case, high power lasers on singlemode fiber are prone to additional problems such as nonlinear noise sources. Since many existing data centers have been planned with much longer link distances, there is expected to be a need for ten and forty gigabit repeaters and channel extenders; however these devices are expensive and must be located at inconvenient places along the link path. It is desirable to overcome the dispersion limit of high speed data links, and thereby achieve much longer working distances without repeaters on singlemode fiber.
In order to compensate for singlemode dispersion, a method is required for narrowing the widths of optical pulses being launched into the fiber, without resorting to a special type of expensive laser device. It is known that launching a Gaussian optical pulse through a Gaussian wavelength selective bandpass filter will reduce the pulse width. There is a tradeoff of pulse width vs. optical power. A higher power transmitter is required, but this can be easily achieved with current transceiver designs simply by increasing the laser bias current. However, it is not practical to implement this tradeoff unless a controlled method exists for matching the center wavelength of an arbitrarily chosen laser to the center of a filter passband. Otherwise, the optical loss between the laser and filter becomes too great and any advantages from reducing the pulse width are lost. One method and system for narrowing the widths of optical pulses is disclosed in copending patent application Ser. No. 09/865,256, for “Apparatus and Method for Wavelength-Locked Loop for Systems and Applications Employing Electromagnetic Signals,” filed May 22, 2001, the disclosure of which is hereby incorporated herein in its entirety by reference. The present invention is an improvement on the system described in this copending application.